feat(dstackup): support configurable release API

[kvinwang]

Summary

• add a global --release-api-base-url option used by both dstackup image pull and install-time automatic image downloads
• preserve the existing version-to-repository routing while allowing plain HTTP proxies and local test servers
• add a GitHub Releases API overlay mock that publishes local assets and relays missing requests upstream
• document the local publish and pull workflow

Testing

• cargo fmt --check
• cargo test -p dstackup --quiet
• python3 -m py_compile tools/mock-github-releases.py
• end-to-end local publish, SHA-256 verification, extraction, explicit-version pull, and latest-version pull

[Copilot]

Pull request overview

Adds a configurable “GitHub-compatible Releases API base URL” to dstackup, so both dstackup image pull and install-time automatic image downloads can be exercised against an HTTP proxy or local test server. It also introduces a small local overlay server to publish/serve release assets while relaying cache misses to the real GitHub API, plus documentation for the workflow.

Changes:

• Add global --release-api-base-url and thread it through install and image pull release lookups.
• Update release-lookup URL construction to use the configured API base (including new unit test coverage for trailing slash handling).
• Add tools/mock-github-releases.py and a doc guide for locally publishing and pulling test releases.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tools/mock-github-releases.py	New local GitHub Releases API overlay for publishing local assets and relaying upstream requests.
dstack/crates/dstackup/src/main.rs	Plumbs the global release API base URL into install and image subcommands.
dstack/crates/dstackup/src/install.rs	Passes the release API base URL into install-time image resolution/pull logic.
dstack/crates/dstackup/src/image.rs	Uses the configured base URL for tagged/latest release API calls and extends tests.
dstack/crates/dstackup/src/cli.rs	Introduces the global --release-api-base-url flag and default value.
docs/testing-dstackup-image-pull.md	Documents running the mock server, publishing a local release, and pulling via the custom base URL.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.